The prosthetic replacement of joints has evolved over the years from early, relatively crude models to current prostheses which closely replicate functions and motions of a natural joint. Some of the joints amenable to prosthetic replacement include the shoulder, hip, knee, ankle and wrist. Prosthetic joints have provided patients with increased comfort, range of motion and nearly normal lifestyles.
Some of the problems encountered with prosthetic joints include excessive wear between components of the prosthetic implants which move with respect to each other. Additionally, movement of the implant with respect to the patient's bone compromises fixation. A third problem is an abnormal stress transference from the implant to the bone.
To address some of the problems of fixing the implant to the bone, cemented implants have been developed. The cement initially acts as a grout to "form fit" the implant to the bone. After the cement cures to a hard material, it becomes mechanically fixed to the bone by interdigitating into the bone trabeculae. Cementing a plastic or metal implant securely to natural bone has greatly improved the status of joint replacement over the prior state of merely implanting the component into the bone and hoping that it actually stayed securely in place. Cement fixation generally provides excellent short term results. However, younger, heavier, or more active individuals may find that the bond between cement and bone eventually breaks down. Consequently, a loosening or separation between the bone and the implant occurs.
To address the problem of cement breakage or implant separation, an alternative is to coat a surface of the implant with a porous material which allows the patient's bone to grow into the pores, thereby mechanically fixing the implant to bone or alternatively coating the implant with a calcium-phosphate type of ceramic which may chemically bond the implant to bone. As a result in either case, the implant is biologically attached to the bone. This "cementless" procedure represents the current state of the art in implantation of joint prostheses. The patient's own tissue eventually holds the implant securely in place, either mechanically or chemically, and the implant subsequently becomes a permanent part of the bone.
Another problem, seen with both cemented and cementless implants, is occasional extensive osteolysis. This osteolysis, or bone dissolution, occurs throughout all areas of the bone into which the stem was implanted. The etiology is possibly a histiocytic foreign body reaction. In the case of cemented devices, the reaction may be from fractured particulate cement and/or secondary to polyethylene particulate wear debris. In the case of cementless stemmed implants, the reaction is felt to be secondary to particulate polyethylene or particulate metal alloy from fretting wear of the implant. Regarding reaction to metal alloy, fretting secondary to micromotion at the implant-bone interface theoretically initiates the foreign body immune response.
A further problem encountered with joint implants is an abnormal stress transference from the implant to the bone. The ideal stress transference of load to the bone is the normal, anatomical transference. To approximate it, the implant material should have mechanical properties similar to those of bone and should replace only the destroyed joint surface. Thus no implant material, or only a minimal amount of implant material, would be placed in the intramedullary canal of the bone. This is difficult to do with implants having porous surfaces. The reason is that such implants require immediate rigid fixation for a sufficient time period, at least six to twelve weeks, to assure bony attachment or chemical bonding. If the device is not held rigidly, micro-motion occurs at the implant-bone interface. The result is a less stable fibrous tissue interface rather than the necessary, more stable, securely-fixed bony attachment.
Currently the most common method of holding the implant rigidly in the bone is by providing the implant with a stem. The stem "press-fits" into the intramedullary cavity of the bone, e.g., the femur. Such a press-fit of the stem into the shaft of the bone holds the device rigidly and allows for an adequate bone attachment or chemical bonding for secure fixation. For the surgeon it also provides the desired proper anatomical placement of the implant in the bone in a reproducible manner. If no intramedullary cavity is available, as in the pelvis for example, the implant is anchored to the bone with a threaded anchor bolt.
The shortcoming of the aforesaid approach is that loading of the bone is no longer physiologic. Instead of the normal loading primarily at the end of the bone near the joint surface, the bone is loaded more distally where the stem of the implant is affixed. The result is an abnormal transference of stress which bypasses or "unloads" the end or joint surface portion of the bone. Consequently that portion of bone undergoes resorption. This leads to weakening over a period of years, thus creating a potential for fracture or resorption of the bone that previously held the implant securely. The result is again a loosening of the implant within the bone with all the adverse consequences previously mentioned.
For implants which are held in place with a screw, such as an acetabular cup of a hip prosthesis, the non-physiologic transference of stresses is less pronounced. This is because the location and orientation of the anchoring bolt can be selected to minimize non-natural load transference stresses. Nevertheless, the potential problems remain; namely, fretting with resultant corrosion and lysis or fatigue failure from micro-motion and eventual fracture of the anchoring bolt.
Because a stem placed down the medullary cavity of the bone produces an abnormal stress distribution, the possibility of using an implant without a stem presents itself. Such an implant would essentially only resurface the destroyed articular surface. This is more readily done in certain joints, such as the knee, elbow, or ankle, than in others, such as the hip, shoulder or wrist.
However, even if a stemless implant is feasible, its immediate rigid fixation is not as secure as if the implant were anchored with a stem or a bolt. Because the stem or bolt functions to align an implant in its correct position until bone attachment is complete, an alternative mechanism is necessary to accomplish these functions if a stemless implant is used. One such mechanism could be transcortical fixation of the implant with multiple screws. This, however, makes it more difficult for the surgeon to correctly and reproducibly position and align the implant. An alternative approach is disclosed and claimed in U.S. Pat. No. 4,990,161 the entire disclosure of which is made part hereof and incorporated herein by reference.